Cost-effective cleavage of carbon-oxygen bonds plays an important role and remains a great challenge in energy production and fine chemicals synthesis. With the increasing demand for fuel and commodity chemicals and our dependence on non-renewable resources like petroleum, the area of catalysis has recently started to develop practical alternative strategies to produce fuels and commodity chemicals from renewable and terrestrially abundant sources such as oxygen-rich biopolymers, i.e., biomass. Current catalytic processes for the reductive upgrading of biomass employ high temperatures, high pressures and high catalyst loadings, compromising precious and semi-precious transition metals. The inherently high oxygen content of biomass poses refining problems due to coke formation under harsh processing conditions, thus, compromising process efficiencies and profitability. The catalytic reduction of lignocellulose, in particular, has been widely studied using conventional cobalt- and nickel-promoted molybdenum and tungsten catalysts used commercially for petroleum hydrotreatment. The heterogeneous nature of these catalysts results in non-selective oxygen extrusion from these diversely functionalized biopolymers through various competing pathways, ultimately affording complicated mixtures of products at low conversion.
Optimal catalysts for biomass refining are systems that afford high conversion under moderate reaction conditions. Reductive scission of C—O bonds under relatively low temperatures will minimize char formation and other competitive thermal reactions. High selectivity for direct C—O bond hydrogenolysis is desired to suppress excessive hydrogen consumption as most heterogeneous catalysts hydrogenate aromatic rings prior to C—O bond scission, producing cycloalkanes and cycloakanols which are lower value commodity chemicals. Moreover, ideal biomass hydrotreatment catalysts should effect tandem depolymerization and refining to convert bio-oils into higher value liquid fuels via side chain removal.
The development of first row transition metal catalysts for hydrotreatment has not received much attention because such compounds are believed to possess low activity. On the contrary, recent results from credible investigators on the nature of inorganic supports and catalyst concentrations suggest that the active catalytic center in commercial cobalt-molybdenum sulfide (CoMoS2) catalysts may well be the cobalt rather than the molybdenum, as is typically thought. Though most lignocellulose refining strategies in the literature are catalyzed by precious second- and third-row metals, it is important to note that the most efficient upgrading technology thus far, is mediated by an FeS catalyst.